Why does sulfuric acid dehydrate

This is a preparation commonly used to illustrate the formation and purification of a liquid product. The fact that the carbon atoms are joined in a ring has no bearing on the chemistry of the reaction. Cyclohexanol is heated with concentrated phosphoric V acid, and the liquid cyclohexene distils off and can be collected and purified. Phosphoric V acid tends to be used instead of sulfuric acid because it is safer and facilitates a less complex reaction.

With more complicated alcohols, the formation of more than one alkene is possible. Butanol is a good example of this, with three different alkenes formed when it is dehydrated. When an alcohol is dehydrated, the -OH group and a hydrogen atom from the next carbon atom in the chain are removed. With molecules like butanol, there are two possibilities for this.

This situation is further complicated by the fact that butene exhibits geometric isomerism ; thus, a mixture of two isomers is formed: cis -butene and trans -butene. The compound cis -butene is also known as Z -butene; trans -butene is also known as E -butene. Which isomer is formed is a matter of chance. Hence, the dehydration of butanol leads to a mixture containing the following compounds:. The carbon will eventually rise to two or three times the height of the beaker.

The steam can be tested with cobalt chloride paper. This will turn from blue to pink. Sulfur dioxide will turn the potassium dichromate paper from orange to blue-green. The beaker itself becomes very hot. One drop of water from a wash bottle squirted on to the outside of the beaker will steam.

Place about 3 g of blue, hydrated copper II sulfate on a watch glass and pour onto it about 2 cm 3 of concentrated sulfuric acid. Over a period of about three minutes, the colour will change from blue to white, as the concentrated sulfuric acid removes the water of crystallisation from the salt. Heat is evolved. The change can be reversed by adding water. This needs to be done very carefully — adding water to acid is not normally good practice.

Teaching notes These demonstrations are a spectacular warning of the danger of handling concentrated sulfuric acid. Additional information This is a resource from the Practical Chemistry project , developed by the Nuffield Foundation and the Royal Society of Chemistry.

Level years years years. Use Demonstrations. Category Acids and bases Redox chemistry Organic chemistry Reactions and synthesis. Experimental skills and strategies carry out experiments appropriately having due regard to the correct manipulation of apparatus, the accuracy of measurements and health and safety considerations WJEC Combined science Working scientifically 2.

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Load more articles. Once one know what it is spoken about, it is more than acceptable to say that reaction between sulphuric acid and, eg, a sugar, is a dehydratation one. Of course, it has nothing to do with a real extraction or with drying. Indeed water isn't there, but it is a dehydratation reaction. Beside this, plus 1 for fixing the thread. Is possible this exothermic nature is just more commonly known for H2SO4, as it is more common acid form?

In a closed dessicator, for example, the acid will be separate from the substance to be dried. All that matters here is that any water vapour in the vessel will be rapidly absorbed by the acid as that is highly favoured by the thermodynamics of water mixing with the acid.

And the low vapour pressure of the acid won't contaminate the substance. The same must be true for nitrogen dioxide. There is indeed something special about sulfuric acid and water combination.

Acids of P V are not particularly strong dehydration agens. Show 2 more comments. If the oxidising nature of nitric acid is the reason we do not use it as dehydrating agent then h2so4 should also not be used. I am not sure about the value of this argument. It is not a strong dehydrating agent. I agree with Maurice. Show 1 more comment. Sign up or log in Sign up using Google.

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